A web offset printing press typically includes a plate cylinder, a blanket cylinder and an impression cylinder supported for rotation in the press. The plate cylinder carries a printing plate having a rigid surface defining an image to be printed. The blanket cylinder carries a printing blanket having a flexible surface which contacts the printing plate at a nip between the plate cylinder and the blanket cylinder. A web to be printed moves through a nip between the blanket cylinder and the impression cylinder. Ink is applied to the surface of the printing plate on the plate cylinder. An inked image is picked up by the printing blanket at the nip between the blanket cylinder and the plate cylinder, and is transferred from the printing blanket to the web at the nip between the blanket cylinder and the impression cylinder. The impression cylinder can be another blanket cylinder for printing on the opposite side of the web.
A conventional printing blanket is manufactured as a flat sheet. Such a printing blanket is mounted on a blanket cylinder by wrapping the sheet around the blanket cylinder and by attaching the opposite ends of the sheet to the blanket cylinder in an axially extending gap in the blanket cylinder. The adjoining opposite ends of the sheet define a gap extending axially along the length of the printing blanket. The gap moves through the nip between the blanket cylinder and the plate cylinder, and also moves through the nip between the blanket cylinder and the impression cylinder, each time the blanket cylinder rotates.
When the leading and trailing edges of the gap at the printing blanket move through the nip between the blanket cylinder and an adjacent cylinder, pressure between the blanket cylinder and the adjacent cylinder is relieved and established, respectively. The repeated relieving and establishing of pressure at the gap causes vibrations and shock loads in the cylinders and throughout the printing press. Such vibrations and shock loads detrimentally affect print quality. For example, at the time that the gap relieves and establishes pressure at the nip between the blanket cylinder and the plate cylinder, printing may be taking place on the web moving through the nip between the blanket cylinder and the impression cylinder. Any movement of the blanket cylinder or the printing blanket caused by the relieving and establishing of pressure at that time can smear the image which is transferred from the printing blanket to the web. Likewise, when the gap in the printing blanket moves through the nip between the blanket cylinder and the impression cylinder, an image being picked up from the printing plate by the printing blanket at the other nip can be smeared. The result of the vibrations and shock loads caused by the gap in the printing blanket has been an undesirably low limit to the speed at which printing presses can be run with acceptable print quality.
Another problem caused by the gap at the adjoining ends of a conventional printing blanket is the circumferentially extending void defined by the width of the gap. The void defined by the width of the gap interrupts and reduces the circumferential length of the printing surface on the blanket cylinder. This causes an area of the web to remain unprinted each time the blanket cylinder rotates. Such unprinted areas of the web reduce productivity and increase waste. In addition, such a conventional printing blanket is not easily properly attached to a blanket cylinder. As a result there can be considerable press downtime, which can be expensive. Furthermore, the blanket cylinder itself must be equipped with means for engaging the opposite ends of the printing blanket to hold them in place.
Another problem associated with conventional printing blankets is caused by the pressure exerted against the flexible surface of the printing blanket by the rigid surface of the printing plate at the nip between the blanket cylinder and the plate cylinder. The flexible surface of the printing blanket is indented by the rigid surface of the printing plate as it is pressed against the printing plate upon movement through the nip. At the center of the nip, the cylindrical contour of the rigid printing plate impresses a corresponding cylindrical depression in the flexible printing blanket. When a depression is pressed into the flexible printing blanket, bulges tend to arise on each of the two opposite sides of the depression. Such bulges appear as standing waves on the surface of the printing blanket on opposite circumferential sides of the nip. A point on the surface of the printing blanket moves up and over such standing waves as it enters and exits the nip. Compared with a point on the rigid cylindrical surface of the printing plate, a point on the flexible surface of the printing blanket traverses a greater distance as it moves past the nip. The speeds of those surfaces therefore differ at the nip. A difference in surface speeds causes slipping between the surfaces which can smear the ink transferred from one surface to the other.
Printing blankets are known to include compressible rubber materials which compress under the pressure exerted against the printing blanket by the printing plate at the nip therebetween. Compression of the printing blanket at the nip reduces the tendency of bulges to form at opposite sides of the nip. Standing waves which could smear the ink on the rotating printing blanket are thus reduced, but repeated compression and expansion of the compressible rubber material can cause the printing blanket to overheat.